In recent years it has been required with ever increasing severity that compact electronic devices such as portable telephones be reduced in size. In such a situation, it has been practice to incorporate circuit elements for constituting a device into a composite laminate device and to mount the composite device on a main board.
As shown in FIGS. 16 and 17, the composite device of the laminate type has a laminate structure comprising a plurality of ceramic layers 1, 2 each of which is provided on the surface thereof with a plurality of circuit element patterns 11 or 21 for constituting an inductor or capacitor. Such circuit element patterns 11, 21 are connected to one another through conductor patterns 13, 23 formed on the ceramic layers 1, 2 or conduction channels (via holes) 12, 22 extending through the ceramic layers 1, 2 to provide an electronic circuit such as a filter.
With such composite devices, it has been proposed to form a pattern (hereinafter referred to as an “L-pattern”) for providing an inductance on the magnetic body ceramic layer 1 so as to increase the inductance of the L-pattern and to form a pattern (hereafter referred to as a “C-pattern”) for constituting a capacitor on the dielectric ceramic layer 2 in order to increase the capacitance of the C-pattern [JP-A No. 60-106114(1985), JP-A No. 6-333743(1994), etc.].
Composite devices of the type mentioned are fabricated generally in the following manner. A required number of magnetic body boards prepared by forming an L-pattern on the surface of each of magnetic body green sheets are laminated to obtain an inductor laminate, and a required number of dielectric boards prepared by forming a C-pattern on the surface of each of dielectric green sheets are laminated to obtain a capacitor laminate. The two laminates as lapped over each other are fired to join the boards and obtain a sintered laminate. A plurality of electronic components are mounted on the surface of the sintered laminate as required to complete a composite device of the laminate type in the form of a single chip.
However, the conventional composite device of the laminate type has the following problem. In firing the combined laminate obtained by laminating the magnetic body boards comprising magnetic body green sheets and the dielectric boards comprising dielectric green sheets, the magnetic body ceramic layer 1 and the dielectric ceramic layer 2 are curved and greatly deflected by the firing step as shown in FIG. 18 because the magnetic body green sheet and the dielectric green sheet differ greatly in shrinkage, possibly creating a crack K in the resulting laminate.
FIG. 19(a) shows the portion A in FIG. 18 on an enlarged scale before firing, and FIG. 19(b) shows the portion A of FIG. 18 on an enlarged scale after firing. At the joint between the ceramic layers 1, 2 lapping over each other as seen in FIG. 19(a), the circuit element patterns 11, 21 and the via holes 12, 22 corresponding thereto are positioned in match before firing, whereas the circuit element patterns 11, 21 are positioned as shifted or separated from the corresponding via holes 12, 22 after firing as shown in FIG. 19(b), hence the problem of a reduced yield.
Accordingly, another composite device of the laminate type has been proposed which comprises a magnetic body ceramic layer, a dielectric ceramic layer, and a material mixture layer formed between the two ceramic layers and containing a mixture of substantially the same magnetic material as the magnetic body ceramic layer and substantially the same dielectric material as the dielectric ceramic layer so as to prevent separation at the joint interface between the ceramic layers [JP-A No. 6-325979(1994)].
With the proposed composite device, the material mixture layer is intermediate between the magnetic body ceramic layer and the dielectric ceramic layer in shrinkage, such that the difference in shrinkage between the material mixture layer and each ceramic layer is diminished to one half the shrinkage difference between the ceramic layers, whereas there still exists a great shrinkage difference between the component layers of the proposed device. Thus, the problem of cracking or separation has yet to be overcome.